Dale Keiger in Johns Hopkins Magazine:
The United States Department of Agriculture (USDA) estimates that livestock and poultry produce 335 million tons of manure per year, which is one way resistant pathogens get out of animals and into the environment. That's 40 times as much fecal waste as humans produce annually. Farms use it for fertilizer and collect it in sheds and manure lagoons, but those containment measures do not prevent infectious microbes from getting into the air, soil, and water. They can be transported off the farms by the animals themselves, houseflies, farm trucks, and farm workers, and by spreading manure on other fields. Out in the environment, they form a sort of bank of genetic material that enables the spread of resistance.
Kellogg Schwab, director of the Johns Hopkins Center for Water and Health, refers to a typical pig farm manure lagoon that he sampled. “There were 10 million E. coli per liter [of sampled waste]. Ten million. And you have a hundred million liters in some of those pits. So you can have trillions of bacteria present, of which 89 percent are resistant to drugs. That's a massive amount that in a rain event can contaminate the environment.”
He adds, “This development of drug resistance scares the hell out of me. If we continue on and we lose the ability to fight these microorganisms, a robust, healthy individual has a chance of dying, where before we would be able to prevent that death.” Schwab says that if he tried, he could not build a better incubator of resistant pathogens than a factory farm.